Patent Literature 1 discloses a double floor structure in which beams (as constituent members for an upper floor) are arranged on support legs, which are extruded shapes of an aluminum alloy and placed on a lower floor. The support legs disclosed in Patent Literature 1 are formed by assembly of upper, intermediate, and lower members, which are extruded shapes of the aluminum alloy. In the case where a double floor structure is constructed by use of the above support legs, it is possible to comply with various requirements from customers and execution conditions at low cost.